Electro-acoustical device for reproducing low pitch notes



Nov. 21, 1961 H. MAYR 3,009,528

ELECTRO-ACOUSTICAL DEVICE FOR REPRODUCING LOW PITCH NOTES Filed July 18, 1957 United States Patent The present invention relates to electroacoustical units and more particularly to loudspeakers for reproducing low pitch notes.

As is well known, conventional loudspeakers are not well adapted to reproduce low pitch notes, the wave 5 lengths of which are greater than the dimensions of the loudspeaker. This is due to the fact that the two faces of the vibrating cone of a conventional loudspeaker act as two sources of sound with opposed phases. If, for instance, the cone moves forward, the front face tends to compress air while the rear face tends to rarify it. The differences of pressure so produced tend to be eliminated, creating only a current of air around the cone of the loudspeaker, and thus no sensible acoustic radiation is produced.

In order to obviate this disadvantage, various solutions have been proposed, one of which involves the separation of the two sources, for instance, by means of a sufliciently extended acoustic screen. Another known method consists of enclosing the loudspeaker in a closed box, so that only the front face of the cone faces the outside. In this latter method, the acoustic radiation of the rear face is completely eliminated, but this is accompanied by a considerable disadvantage for, in 'order to move, the cone now must compress or expand the air enclosed in the box. This air acts as a spring that tends to prevent movement of the cone particularly at low frequencies and acceptable results are obtained only if the box is rather voluminous.

A better system, which is also well-known, is to use a box inwhich there are provided apertures of suitable dimensions so that air can leave the box if compressed or can enter if the air inside of the box is rarified. The air passing through the apertures opposes movement due to inertia of its mass and forms a resonant system with the air in the interior of the box, which reacts against compression like a spring. Ordinarily the frequency of that system is made to coincide with that of the associated loudspeaker.

In such a device, for frequencies higher than the resonance, the reaction of the air in the apertures overcomes that of the air in the box. There then occurs considerable compression in the interior of the box, and a limited flow of air from the apertures. The movement of air in the apertures is in opposite direction with respect to that in the interior of the cone such that, for example, if the cone moves inwards air is sucked into the interior through the apertures, or vice versa. Hence the apertures behave like a second source of sound reinforcing the loudspeaker. At frequencies lower than resonance, the movement of air in'the apertures reverses the phase and opposes the sound-effect of the loudspeaker. At frequencies higher than resonance, the movement of air in the apertures reverses the phase and opposes the sound-effect of the loudspeaker. Consequently, sound radiation below the frequency of resonance turns'out to be rather weak, although still better than that of the loudspeaker itself.

In the useful range, that is, about from resonance up,

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and of which one is formed by the external face of the loudspeaker, while the other one is formed by the air leaving the apertures, which in turn originates at the inner face of the loudspeaker.

The main defect of that system is the considerable distance between the apertures and cone, so that the movement of air in the apertures is no longer in phase with the external face of the cone. Moreover, sound is radiated in part by the apertures and in part by the loudspeaker; the two parts must compound with each other in external space to yield the complete and entire sound. The reproduction obtained is very poor.

It is an object of the present invention to eliminate completely the above described disadvantages by providing a particularly simple device which, without involving the addition of any auxiliary means enables optimizing the acoustical efficiency of the loudspeaker. According to the invention the loudspeaker is assembled in a completely closed box which leaves only the front face of the cone free. In the cone of the loudspeaker itself there are provided one or more apertures which permit communication of the air enclosed in the box with the external space. The number of those apertures and their area are chosen preferably in such way as to tune the box approximately with the resonant frequency of the loudspeaker itself and to avoid weakening the cone too much which latter advantage may be achieved by using a great number of small apertures in lieu of few large apertures.

The operation and characteristics of the device will be illustrated with reference to the accompanying drawing which shows a preferred embodiment of the'invention. In the drawing:

FIGURES l and 2 illustrate a section and front view of a device provided with a magnetodynamic loudspeaker.

FIG. 3 is a variant of the preceding figures.

In FIGS. 1 and 2 is illustrated an electroacoustical device comprising a conical membrane 3 including a peripheral portion 3a and a central portion 31;. This device further includes in the central portion 13b means "4 responsive to magnetic variations for being vibrated. In addition, magnetic means 5 are operatively positioned with respect to means 4 to produce the magnetic variations necessary to vibrate the latter. A support or casing 1 is provided to support the membrane 3 by means of the peripheral portion 3a thereof. The membrane defines at least one aperture 4' for the passage of air through said membrane. The aperture is spaced from 0 the means 4 by a distance which is less than one-half of there are thus two sources which enhance each other,

the wavelength of the lowest frequency to be reproduced.

It will be noted that the membrane 3 divides the casing 1 into two chambers 6 and 7 which chambers are connected through the apertures '4'.

For frequencies higher than the resonance of the box, the mechanical reactance due to the inertia of the air in the apertures is higher than that due to the rigidity of v the air in the box. As a consequence, if the cone of the loudspeaker moves towards the interior of the box, the air contained therein is compressed. This pressure acts upon the walls of the box, including the loudspeaker cone and, therefore, also upon the air in the apertures, and tries to expel it. Due to the inertia of the air in the apertures, however, this air assumes such a movement that its acceleration is directed towards the exterior. For an oscillatory movement as is verified in acoustics, this corresponds to a movement of direction opposed to acceleration and is directed towards the interior. Hence if the cone moves towards the interior of the box, the air in the holes is sucked towards the interior, and

if the cone moves towards the exterior, the air is blown 3 from the holes towards the outside. The air passing through the holes assists and enhances, therefore, the sound-action of the loudspeaker.

At the frequency of resonance, the reactance of the air in the apertures is equal to the reactance of the air in the box, with the result that the least movement of the cone causes a flow of air in the apertures, together with a strong pressure upon the cone that opposes its movement. The overall result is that the cone remains nearly motionless, while the role of a source of sound is assumed by the air in the apertures.

By making the resonance of the box coincide with that of the loudspeaker, one thus also obtains the very useful effect of damping the otherwise violent oscillations of the cone at that frequency and of limiting the amplitude thereof, which also reduces distortion.

For frequencies lower than the resonance of the box, the reactance of the air in the apertures is smaller than that of the air in the box. As a consequence, for a movement of the cone towards the interior, the air in the box is slightly compressed, but leaves from the apertures with a movement opposite that of the air displaced by said cone.

Owing to the residue of compression in the interior, the volume of air leaving the apertures is greater than that of the air displaced by the cone. Hence the apertures are more effective as a source of sound than is the cone.

In the apertures, there is thus obtained a second source of sound, which for frequencies higher than resonance reinforces the action of the cone as well as which for frequencies lower than resonance contributes effectively to radiation of sound. At the resonance frequency, a very useful damping of the oscillations of said cone is obtained.

The particular effectiveness of the apertures in the cone according to the invention lies in that the two sources of sound, that is, the cone itself and the apertures, cooperate positively, and in that the air emitted by the apertures joins immediately that displaced by the cone to form in practice a single source of sound to the end of optimum sound-radiation. All the obnoxious effects of non-uniformity of the sound-field are prevented, as might otherwise occur in the case of multiple sources.

The shape and other particulars indicated in the drawings may vary without departing from the scope of the present invention. For example, the apertures may be advantageously arranged at approximately equal distance between the apex of the cone and its border, said distance being that measured along a generating line. In some instance the arrangement indicated in FIG. 3 may be useful, where 4 indicates the apertures. These are provided in the corrugated border of the cone, to obtain a considerable reduction of the stiffness of the elastic suspension, while keeping all the advantages of the apertures described above.

What is claimed is:

An electro-acoustical device comprising a conical membrane for reproducing a range of audio frequencies including peripheral and central portions and further including, in the central portion, means responsive to magnetic variations for being vibrated, magnetic means operatively positioned with respect to the first said means and adapted for producing the magnetic variations, and a support supporting the membrane by means of said peripheral portion, said support defining a chamber in which said conical membrane is mounted and having a single opening substantially closed by said membrane, said membrane defining apertures for the passage of air from said chamber, said apertures having an area whercby the resonant frequency of the chamber is matched to that of said membrane, said membrane and magnetic means being supported on said support adjacent said opening.

References Cited in the file of this patent UNITED STATES PATENTS 1,869,178 Thuras July 26, 1932 1,997,790 Heidrich Apr. 16, 1935 2,022,060 Swickard Nov. 26, 1935 2,122,587 Ringel July 5, 1938 2,712,360 Reisz July 5, '1955 2,732,908 Brittain Jan. 31, 1956 2,766,839 Baruch et al. Oct. 16, 1956 2,845,135 Cohen et al. July 29, 1958 FOREIGN PATENTS 256,395 Great Britain Aug. 12, 1926 119,647 Australia Feb. 26, 1945 

